Article http://msdn.microsoft.com/msdnmag/issues/03/07/NET/ ("Calling Win32 DLLs in C# with P/Invoke") gives a helpful wrapper class for MessageBeep.

///     processes. The 32-bit and 64-bit DLLs must have different names.

///     Windows Store app development If dwThreadId is zero, then window hook DLLs are not loaded in-process for the

    ///Application file CLSIDs or IIDs do not match corresponding DLLs

    [Description("Application file CLSIDs or IIDs do not match corresponding DLLs")]

    '''Application file CLSIDs or IIDs do not match corresponding DLLs

    <Description("Application file CLSIDs or IIDs do not match corresponding DLLs")> _

        /// <summary> 2705 - The password does not meet the requirements of the password filter DLLs. </summary>

        /// Application file CLSIDs or IIDs do not match corresponding DLLs

    ''' The RVA of the first code byte in the file that will be executed. For DLLs,  

    ''' be set to 0 in DLLs, and none of the previous notifications will be received.

    ''' For EXEs, the default ImageBase is 0x400000. For DLLs, it's 0x10000000.  

    ''' system DLLs. Otherwise, this field can be 0. The checksum is placed in the  

    ' address (RVA) of the first code byte in the file that will be executed.)  For DLLs, this

    ' This field can be set to 0 in DLLs, and none of the previous notifications will be received.

    ' step of applying base relocations. The default ImageBase is for EXEs 0x400000, for DLLs 0x10000000.

    ' Checksums are required for kernel-mode drivers and some system DLLs.  Otherwise, this field

    ' address (RVA) of the first code byte in the file that will be executed.)  For DLLs, this

    ' This field can be set to 0 in DLLs, and none of the previous notifications will be received.

    ' Checksums are required for kernel-mode drivers and some system DLLs.  Otherwise, this field

Unloads any DLLs that are no longer in use and whose unload delay has expired.

     Setting unloadDelay=0 forces the unloading of any DLLs without any delay.

public static void FreeDllsNow(UInt32 pause)

Unloads any DLLs that are no longer in use and whose unload delay has expired.

     Setting unloadDelay=0 forces the unloading of any DLLs without any delay.

public static void FreeDllsNow(UInt32 pause)

     In addition all use of function dlls requires marshaling

Some DLLs only export by ordinal. Because you can't express this mode of GetProcAddress to the marshaller, one possibility is to import the function a second time with another name, such as:

     //set the dll path so it can find the dlls